1. Field of the Invention
The present invention relates to a full color organic electroluminescence display device, wherein an electron transporting layer, which is applied to organic layers as a common layer, is formed with a first thickness in the red and green emission layer regions and with a second thickness, which is different from the first thickness, in the blue emission layer region so that it has a high purity of color, as well as improved red and green color efficiency.
2. Description of the Related Art
In general, an organic electroluminescence display device realizes colors based on the principle that holes and electrons, which are injected from an anode and a cathode, are recombined in an emission layer to emit light. Thus, it has a layered structure having an emission layer inserted between the anode and the cathode. However, such a structure cannot provide high luminous efficiency. Accordingly, an electron injecting layer, an electron transporting layer, a hole transporting layer, a hole injecting layer, and the like are selectively added and inserted between respective electrodes and emission layers.
Specifically, a full color organic electroluminescence display device has an emission layer, on which red (R), green (G), and blue (B) pixelregions are formed, to implement full color. Although various emission substances have been developed to implement full color, and can exhibit a high purity of color, there have been found few red, green and blue emission substances which are suitable for full color organic electroluminescence display devices and thus, research continues to develop new substances. As a result of research on the structural aspects of organic electroluminescence display devices for implementing full color, it has also been suggested to use a color conversion layer or a color filter in the organic electroluminescence display device.
However, there has been no organic electroluminescence display device which can implement full color in a fully satisfactory level. This is due to a practical difficulty in forming a fine pattern of an organic thin film, such as an emission layer, an electron injecting layer, and a hole transporting layer, as well as due to difficulty in obtaining the high purity of color due to the differences in luminous efficiency between red, green and blue colors depending on the layers.
FIG. 1 is a cross-sectional view showing the structure of a conventional full color organic electroluminescence display device.
Referring to FIG. 1, anode electrodes 12 are deposited and patterned on a substrate 11. The anode electrodes 12 define pixel regions R, G, B using insulating layers 13. As organic layers, hole transporting layers 21 are formed on the pixel regions R, G, B by, for example, vacuum deposition. Red, green and blue emission substances are deposited on the hole transporting layers 21 and patterned to form red, green and blue emission layers 22R, 22G, 22B. Electron transporting layers 23 are then formed on the red, green and blue emission layers 22R, 22G, 22B. Finally, a cathode electrode 31 is deposited on the entire surface of the substrate to complete a full color organic electroluminescence display device.
The full color organic electroluminescence display device, as configured above, has a structure in which its anode electrodes 12; hole transporting layers 21; red, green and blue emission layers 22R, 22G, 22B; electron transporting layers 23; and cathode electrode 31 are isolated in respective color regions by the insulating layers 13. To manufacture a full color organic electroluminescence display device with such a structure, however, the hole transporting layers 21 and the electron transporting layers 23 should be separately formed for each respective color through a number of processes.
In an effort to solve such a problem, U.S. Pat. No. 6,281,634 (the disclosure of which is hereby incorporated herein by reference in its entirety) discloses a method wherein the anodes and the red, green and blue emission layers are separately formed for each respective color, while an electron transporting layer, a hole transporting layer, and a cathode are formed as common layers on the entire surface of the substrate.
FIG. 2 is a cross-sectional view showing the structure of the full color organic electroluminescence display device disclosed in the above U.S. patent.
Referring to FIG. 2, the full color organic electroluminescence display device has a structure in which, on a substrate 11, first electrode layers 42, which act as anode electrodes; a hole transporting layer 51; red, green and blue emission layers 52R, 52G, and 52B; an electron transporting layer 53; and a second electrode layer 61, which acts as a cathode electrode, are formed successively. The hole transporting layer 51 and the electron transporting layer 53 are formed as common layers through the entire substrate.
The full color organic electroluminescence display device, as configured above, is manufactured as follows: the first electrode layers 42 are formed on the substrate 11 through patterning; pixel regions R, G, and B are defined using insulating layers 43; a hole transporting layer 51 is formed as a common layer on the first electrode layers 42 on the entire surface of the substrate; red, green and blue emission layers 52R, 52G, and 52B are patterned and stacked on the hole transporting layer 51; an electron transporting layer 53 is formed as a common layer on the red, green and blue emission layers 52R, 52G, and 52B; and finally a second electrode layer 61 is formed on the entire surface of the substrate. The method disclosed in the above patent is advantageous in that, contrary to conventional methods where the hole transporting layers 21 and the electron transporting layers 23 are formed in an isolated structure, the hole transporting layer 51 and the electron transporting layer 53 are applied as common layers. This simplifies the manufacturing process.
Such an application of the electron transporting layer 53 as a common layer, however, requires consideration on the efficiency and chromaticity coordinates of the red, green and blue emission layers 52R, 52G, and 52B. As is known in the industry, an interference phenomenon between light which is directly emitted via the emission layers 52R, 52G, and 52B, and light which is reflected by an electrode, exhibits excellent chromaticity coordinates from an optical point of view when an organic layer has a very small thickness or a specific thickness of 100 nm or more. However, if the electron transporting layer 53 has a large thickness the driving voltage increases, and thus, the optimal thickness for the electron transporting layer for the blue color is fairly thin.
Such a structure, which is optimized for blue color, is related with the blue emission substance. Specifically, although conventional phosphorescent emission substances may be used for red and green emission, they have too low a luminous efficiency to be used for blue emission and thus cannot be used for a high purity of color. Accordingly, fluorescent substances, which emit light in a manner different from that of the phosphorescent emission substances, are mainly used. A hole blocking layer (not shown) may be formed on the emission layers using a substance, which is similar to the electron transporting layer 53, to block any movement of holes, so that the full color organic electroluminescence display device, which includes emission layers 52R, 52G, and 52B as shown above, can exhibit high purity of color. If the hole blocking layer is formed on the blue emission substance, however, the blue luminous efficiency is deteriorated. Accordingly, the hole blocking layer should be formed except on the blue emission region B.
As such, the blue color has difficulty exhibiting high purity of color, unlike the other colors, and is sensitive to the electron transporting layer 53, which is stacked on the blue emission region B. Accordingly, the thickness of the electron transporting layer 53 of the full color organic electroluminescence display device should be determined in consideration of the blue emission substance. This results in a problem in that it is difficult to design a structure which is both optimized for the red and green emission layers 52R and 52G, and to maximize the efficiency characteristics of red and green colors.
For a full color organic electroluminescence display device with a structure as shown in FIG. 1, the electron transporting layers 23 are separately stacked on respective pixel regions R, G, and B. As a result, the device can be optimized for the characteristics of the emission layers 22R, 22G, and 22B. However, in the case of the full color organic electroluminescence display device, which has a structure as shown in FIG. 2, the electron transporting layer 53 is stacked as a common layer on respective emission layers 52R, 52G, and 52B with the same thickness. Therefore, the luminous efficiency of each of the emission layers may be deteriorated, due to the thickness of the electron transporting layer 53.